Title : Plant responses to water deficit
Light and oxygen are essential to life but can also damage plant photosynthesis and yield, especially the excess of light above 600 to 800 µmol m-2 s-1 in the presence of O2 (a strong oxidant). This metabolic phenomenon, i.e., photoinhibition, is the light-induced decrease in photosystems activity. Mainly associated with other environmental stresses, such as drought, which reduces the CO2 assimilation rate (A). Under high photosynthetic photon flux density (PPFD), the photosynthetic apparatus's ability to utilize the absorbed energy thoroughly is exceeded, but water photolysis continues. In this case, the electrons of water photolysis cannot be assimilated by the photosystem and react with O2 forming reactive oxygen species (ROS), causing damage to membrane integrity. Since, in the field, drought is generally associated with a high PPFD due to a clear sky, photoinhibition an oxidative stress will occur even under moderate water stress. Water deficit induces several physiological responses, varying for the specie studied in its growth stage and cultivation conditions. There is a reduction in A in mild water deficiency, but it is still under debate about the causes of this reduction. Some authors state that there is only a limitation of CO2 substrate (diffusional limitation) due to stomatal closure under mild stress. However, studies conducted from the 1970' showed a diffusional and a metabolic effect on A under mild stress, probably caused by photoinhibition. Moreover, RuBP regeneration is reduced in the Calvin cycle, probably due to a lower chloroplastic ATPase activity or lower inorganic phosphorus (Pi) availability for ATP synthesis. In addition, Pi is exchanged by triose-P from the chloroplast through the phosphate antiporter translocators. Hence, a foliar spray of mono ammonium phosphate (MAP) can improve this cytoplasmatic Pi content and reduce drought's photoinibitory effect. In an experiment with common bean, two days after the foliar Pi spray (MAP), a mild water deficit was imposed, and A was evaluated during water stress and recovery. After rehydration, A and gs of genotypes supplied with Pi were higher than those of non-Pi-supplied plants. These results revealed an up-regulation of A's recovery after water deficit with a foliar spray of Pi. Recently, another study with water deficit imposition in Phaseolus vulgaris L. genotypes was conducted with two experiments, one under a PPFD of 500 µmol m-2 s-1 and the other under 850 µmol m-2 s-1, which showed a more significant decrease in A under mild water stress than under low PPFD. The chlorophyll a fluorescence parameter maximum quantum yield of photosystem II (Fv/Fm), under lower PPFD, was reduced only under severe water deficit. However, Fv/Fm and other parameters were reduced from the beginning of the drought in the second experiment under the higher PPFD. Besides, the non-photochemical quenching (NPQ) values were almost double in the second experiment compared with the first one indicating a higher energy dissipation, primarily by heat, to avoid more intense photoinhibition in the second experiment.